1. Field of the Invention
The invention relates generally to angular measuring systems, and more particularly to a phase-analog detector for providing a digital readout indicative of the angular displacement of a rotary inductive sensor.
2. Description of the Prior Art
Accurate measurement of angular displacement is currently required in many military, aerospace, satellite, radar and fire control systems, where the electrical output signals are required to drive numerical readout displays, provide servo loop feedback signals, and generate computer input data. Among the methods which have found wide application for precise angular measurements are optical encoders, which, for example, may have a pattern impressed upon a disk which is used in conjunction with a light source and a sensor to provide a digital output that is generated when the disk is rotated. Very high resolutions and accuracies are available, but such devices are prohibitively expensive and lack sufficient ruggedness in many applications.
A second approach is the use of a resolver, which is a rotating transformer which provides output analog voltages that are uniquely related to the input shaft angle. Such a resolver is comprised of two orthogonal stator windings and a rotor which is coupled to the input shaft. It provides an absolute indication of position from 0.degree. to 360.degree. of rotation. Two or more resolvers, each yielding data over a unique but limited range, may have their outputs combined in a multi-resolver configuration to yield an absolute indication of greater resolution through 360.degree. of rotation. A resolver is a robust mechanical device that can be exposed to extreme environments without damage or loss of accuracy. As a transformer device, it provides signal isolation and a common-mode rejection to electrical interference. Since it is an analog device, only four wires are necessary for angular data transmission.
Transducers are also available which operate on the principles of inductive or capacitive coupling between conductive patterns bonded to a rigid substrate. Since, as in the resolver, there are no contacting elements except for slip rings, they provide high reliability and maintain original accuracy indefinitely. One such device is the INDUCTOSYN.RTM. position transducer. These transducers are available in both linear and rotary form for a wide range of applications. The term "resolver" is defined herein to include conventional resolvers, inductive and capacitive transducers, and similar devices.
Two methods have been used with a resolver to obtain output voltages proportional to the shaft angle. In the first method, an alternating current is applied to excite the rotor winding and outputs are taken from the two stator windings. Since the stator windings are orthogonally disposed, the output signal amplitudes are related by the trigonometric sine and cosine of the angular shaft displacement. Both stator output signals will have the same phase as the original excitation signal, while their amplitudes are modulated respectively by the sine and cosine functions as the shaft rotates. The ratio of the output amplitudes may then be compared to provide an output signal which provides a high degree of noise immunity. By applying the resultant signal to an amplitude tracking loop, the output may be made to follow automatically the input up to a specified maximum tracking rate. In this application the device is called a tracking converter.
In some systems, however, it is desired to produce a phase-modulated signal because it may conveniently be utilized for both rate and position control in a phase-locked loop. Thus the second method applies two signals in phase quadrature to the respective stator windings. The voltage induced in the rotor when the shaft is displaced angularly has a constant amplitude and frequency, but a phase varying with shaft angle. Thus, when the rotor windings are aligned with the first stator winding, the rotor output signal will be at 0.degree. phase shift, while when the rotor windings align with the second stator winding, the output will be at a maximum of 90.degree. phase shift. At angles between 0.degree. and 90.degree., the phase of the output signal varies substantially linearly with the angle of displacement. As the rotor rotates through 360.degree., the phase of the output rotor signal also varies from 0.degree. to 360.degree. and back to 0.degree..
One technique for converting the phase-modulated signals into digital position data is known as the phase counting scheme, and is based on the direct measurement of phase angle by means of gating a counter with the phase-modulated position signal. Thus, a zero-crossing detector provides an output corresponding to an applied sinosoidal excitation signal and also to the zero-crossing of the rotor signal. The time interval between the two zero crossings is used to gate a pulse generator, which is applied to a counter to provide a digital readout. Because the phase output is sampled, and produces only one position measurement per excitation cycle, a low excitation frequency, such as 400 Hz, as used by many standard resolvers, results in a measurement delay as long as 2.5 ms. Moreover, since this reading also takes time to process, a delay of as much as 3.75 ms may result. This is not acceptable for many high-bandwidth position control servo loops, since these delay variations are a destabilizing influence on the control loop. Further since only one measurement is made per excitation cycle, the resolution of the converter is limited by the frequency of the excitation signal, and the accuracy is limited by the accuracy of the zero-crossing detector.
The present invention provides improved performance by utilizing the stability obtained in state of the art frequency synthesizers to generate highly accurate and stable frequency independent phase-tracking signals to provide a digital output corresponding to the angle of rotation of a resolver. It provides an apparatus for measuring angular displacement by continuously tracking the input signal and applying the phase modulated output derived from the rotor of a resolver whose stator windings are excited in quadrature to a phase tracking loop, and provides a digitized output. It affords high accuracy with minimal measurement delays. Measurement accuracy is independent of excitation frequency and less sensitive to incoherent noise sources.